The present invention relates generally to ventilators for supporting ventilation in air breathing animals. More particularly, the present invention relates to high frequency ventilators which operate by oscillating respiratory air supplied to a subject at a frequency above the normal breathing frequency of the subject.
2. Discussion of Related Art
The use of a medical apparatus to facilitate breathing in mammals is well known in the art. The apparatus may take the form of a simple oxygen mask or tent which supplies oxygen at slightly above atmospheric pressure. Such devices merely assist a person to breather and work with the person's lungs.
Ventilators which operate at high frequency have been suggested in the past. There are two types of high frequency ventilators known in the art. One type, as exemplified by U.S. Pat. No. 2,918,917 to Emerson, employs a reciprocating diaphragm to vibrate a column of gas supplied to a subject. The vibration is in addition to the subject's respiration, natural or artificial, and at a much more rapid rate, for example, from 100 to more than 1500 vibrations per minute. The Emerson apparatus is primarily designed to vibrate the patient's airway and organs associated therewith, although Emerson also recognized that high frequency vibration causes the gas to diffuse more rapidly within the airway and therefore aids the breathing function. However, the Emerson apparatus is capable of supporting the patient's full ventilation and must be used in conjunction with the patient's spontaneous breathing or with another apparatus which produces artificially induced inhalation and exhalation.
The second type of high frequency ventilator is the jet pulse ventilator as exemplified in Schwanbom et al. U.S. Pat. No. 4,265,237. The Schwanbom et al ventilator produces high frequency, high pressure pulses of air which are capable of fully ventilating a patient. The respiration pulse enters with a pressure of 0.2 bar (209 cmH.sub.2 O) to 2.7 bar (2797.2 cmH.sub.2 O). This pressure is sufficient to expand the lungs during inspiration. Expiration is caused by the natural compliance of the lungs after the jet of air is stopped. Accordingly, it can be seen that Schwanbom et al must rely on the compliance of the lungs in order to fully ventilate the patient. If the lung compliance is low, greater pressure must be used. Schwanbom et al also supply a source of lower pressure gas for spontaneous breathing by the patient. While such jet pulse ventilators are useful for some applications, they are not generally applicable and their use is limited mostly to experimental work.
U.S. Pat. No. 4,155,356 to Venegas discloses a respiration assisting apparatus using high frequency pulses to hold a patient's airway open while the patient is breathing or being ventilated with a volume respirator. As with the Emerson device, Venegas is not capable of fully ventilating a subject and must rely either on the natural respiration cycle or on a volume type respirator to sustain the subject.
It is believed that normal breathing functions of air breathing animals are caused by expansion of the chest cavity. The expansion puts a negative pressure on the outside of the plurality of alveolar sacs in the lungs. The innumerable alveolar sacs receive air from the tidal flow or air movements generated, replenishing the sacs with oxygen containing gas and removing carbon dioxide containing gas. Normal breathing produces slight pressure differentials on the alveolar sacs to provide the breathing function. The compliance of the sacs causes them to inflate and deflate in response to the pressure changes.
When the chest cavity expands and creates a negative pressure on the outside of the alveolar sacs, it is believed this causes the sacs to inflate and provides movement of air into the alveolar sacs due to the pressure change. In order to exhale, the pressure on the outside of the alveolar sacs is increased by relaxing the chest cavity, causing the elastic alveolar sacs to collapse and allowing expiration.
As far as is known, commercially available prior art ventilators use a high positive air pressure to inflate the lungs like a balloon. If too much pressure is utilized, the compliance or elasticity of the alveolar sacs is reduced. Eventually, the damage will become so extensive that the sacs will no longer function to expel gas and thereby provide oxygen and carbon dioxide exchange.
When a person is hooked up to ventilator monitoring of blood gases is used to determine whether or not sufficient oxygen exchange is occurring in the alveolar sacs. When the blood gases deteriorate, present ventilators must correct the problem by increasing the pressure of the gas flowing into the lungs. The increase in pressure affects the compliance and elasticity of the sacs even more and can eventually destroy the lungs. A person essentially become addicted to a ventilator and must gradually be weaned from the ventilator.
When there is no lung disease one can use a low pressure with a respirator, because the lungs can breathe on their own to provide the exchange of gases in the alveolar sacs. When there is lung disease present, it may not be possible for the lungs to provide adequate ventilation or gas exchange in the alveolar sacs. This requires some means of facilitating the gas exchange.
The failure of ventilation in conventionally available ventilators generally begins with expiration failures. As mentioned earlier, the conventional method for increasing gas exchange when blood gases deteriorate is to increase the presence of the gas flowing into the lungs. The lungs can sustain a slight over pressuring for a short period of time and not incur permanent damage. However, continued over pressuring will cause a change in the compliance of the alveolar sacs. A bleb or rupture can occur when the alveolar sac has exceeded its elastic limit. Hemorrhaging may result, which destroys the ability of the sac to effect gas exchange and may cause other complications.
During normal breathing, it is believed that the alveolar sacs gradually deflate until they are no longer providing adequate gas exchange. In order to reinflate the alveolar sacs, an individual must sigh, reinflating the alveolar sacs to their full size. Failure to periodically sigh can be fatal because normal breaths allow the alveolar sacs to slowly deflate.